Features • High-performance, Low-power AVR® 8-bit Microcontroller • Advanced RISC Architecture – 130 Powerful Instructions – Most Single-clock Cycle Execution – 32 x 8 General Purpose Working Registers – Fully Static Operation – Up to 16 MIPS Throughput at 16 MHz – On-chip 2-cycle Multiplier • High Endurance Non-volatile Memory segments – 8K Bytes of In-System Self-programmable Flash program memory – 512 Bytes EEPROM – 1K Byte Internal SRAM – Write/Erase Cycles: 10,000 Flash/100,000 EEPROM – Data retention: 20 years at 85°C/100 years at 25°C(1) – Optional Boot Code Section with Independent Lock Bits In-System Programming by On-chip Boot Program True Read-While-Write Operation – Programming Lock for Software Security • Peripheral Features – Two 8-bit Timer/Counters with Separate Prescaler, one Compare Mode – One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture Mode – Real Time Counter with Separate Oscillator – Three PWM Channels – 8-channel ADC in TQFP and QFN/MLF package Eight Channels 10-bit Accuracy – 6-channel ADC in PDIP package Six Channels 10-bit Accuracy – Byte-oriented Two-wire Serial Interface – Programmable Serial USART – Master/Slave SPI Serial Interface – Programmable Watchdog Timer with Separate On-chip Oscillator – On-chip Analog Comparator • Special Microcontroller Features – Power-on Reset and Programmable Brown-out Detection – Internal Calibrated RC Oscillator – External and Internal Interrupt Sources – Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, and Standby • I/O and Packages – 23 Programmable I/O Lines – 28-lead PDIP, 32-lead TQFP, and 32-pad QFN/MLF • Operating Voltages – 2.7 - 5.5V (ATmega8L) – 4.5 - 5.5V (ATmega8) • Speed Grades – 0 - 8 MHz (ATmega8L) – 0 - 16 MHz (ATmega8) • Power Consumption at 4 Mhz, 3V, 25°C – Active: 3.6 mA – Idle Mode: 1.0 mA – Power-down Mode: 0.5 µA 8-bit with 8K Bytes In-System Programmable Flash ATmega8 ATmega8L Pin Configurations PDIP (RESET) PC6 1 (RXD) PD0 2 (TXD) PD1 3 (INT0) PD2 4 (INT1) PD3 5 (XCK/T0) PD4 6 VCC 7 GND 8 (XTAL1/TOSC1) PB6 9 (XTAL2/TOSC2) PB7 10 (T1) PD5 11 (AIN0) PD6 12 (AIN1) PD7 13 (ICP1) PB0 14 28 PC5 (ADC5/SCL) 27 PC4 (ADC4/SDA) 26 PC3 (ADC3) 25 PC2 (ADC2) 24 PC1 (ADC1) 23 PC0 (ADC0) 22 GND 21 AREF 20 AVCC 19 PB5 (SCK) 18 PB4 (MISO) 17 PB3 (MOSI/OC2) 16 PB2 (SS/OC1B) 15 PB1 (OC1A) TQFP Top View 32 PD2 (INT0) 31 PD1 (TXD) 30 PD0 (RXD) 29 PC6 (RESET) 28 PC5 (ADC5/SCL) 27 PC4 (ADC4/SDA) 26 PC3 (ADC3) 25 PC2 (ADC2) (INT1) PD3 1 (XCK/T0) PD4 2 GND 3 VCC 4 GND 5 VCC 6 (XTAL1/TOSC1) PB6 7 (XTAL2/TOSC2) PB7 8 24 PC1 (ADC1) 23 PC0 (ADC0) 22 ADC7 21 GND 20 AREF 19 ADC6 18 AVCC 17 PB5 (SCK) (T1) PD5 9 (AIN0) PD6 10 (AIN1) PD7 11 (ICP1) PB0 12 (OC1A) PB1 13 (SS/OC1B) PB2 14 (MOSI/OC2) PB3 15 (MISO) PB4 16 MLF Top View 32 PD2 (INT0) 31 PD1 (TXD) 30 PD0 (RXD) 29 PC6 (RESET) 28 PC5 (ADC5/SCL) 27 PC4 (ADC4/SDA) 26 PC3 (ADC3) 25 PC2 (ADC2) (INT1) PD3 1 (XCK/T0) PD4 2 GND 3 VCC 4 GND 5 VCC 6 (XTAL1/TOSC1) PB6 7 (XTAL2/TOSC2) PB7 8 2 ATmega8(L) (T1) PD5 9 (AIN0) PD6 10 (AIN1) PD7 11 (ICP1) PB0 12 (OC1A) PB1 13 (SS/OC1B) PB2 14 (MOSI/OC2) PB3 15 (MISO) PB4 16 24 PC1 (ADC1) 23 PC0 (ADC0) 22 ADC7 21 GND 20 AREF 19 ADC6 18 AVCC 17 PB5 (SCK) NOTE: The large center pad underneath the MLF packages is made of metal and internally connected to GND. It should be soldered or glued to the PCB to ensure good mechanical stability. If the center pad is left unconneted, the package might loosen from the PCB. 2486T–AVR–05/08 ATmega8(L) Overview Block Diagram The ATmega8 is a low-power CMOS 8-bit microcontroller based on the AVR RISC architecture. By executing powerful instructions in a single clock cycle, the ATmega8 achieves throughputs approaching 1 MIPS per MHz, allowing the system designer to optimize power consumption versus processing speed. Figure 1. Block Diagram RESET VCC PC0 - PC6 PB0 - PB7 XTAL1 XTAL2 PORTC DRIVERS/BUFFERS PORTB DRIVERS/BUFFERS GND PORTC DIGITAL INTERFACE PORTB DIGITAL INTERFACE AGND AREF MUX & ADC ADC INTERFACE PROGRAM COUNTER STACK POINTER PROGRAM FLASH SRAM INSTRUCTION REGISTER INSTRUCTION DECODER CONTROL LINES AVR CPU GENERAL PURPOSE REGISTERS X Y Z ALU STATUS REGISTER PROGRAMMING LOGIC SPI + COMP. - INTERFACE TWI TIMERS/ COUNTERS OSCILLATOR INTERNAL OSCILLATOR WATCHDOG TIMER MCU CTRL. & TIMING INTERRUPT UNIT OSCILLATOR EEPROM USART PORTD DIGITAL INTERFACE PORTD DRIVERS/BUFFERS PD0 - PD7 3 2486T–AVR–05/08 Disclaimer The AVR core combines a rich instruction set with 32 general purpose working registers. All the 32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers. The ATmega8 provides the following features: 8K bytes of In-System Programmable Flash with Read-While-Write capabilities, 512 bytes of EEPROM, 1K byte of SRAM, 23 general purpose I/O lines, 32 general purpose working registers, three flexible Timer/Counters with compare modes, internal and external interrupts, a serial programmable USART, a byte oriented Twowire Serial Interface, a 6-channel ADC (eight channels in TQFP and QFN/MLF packages) with 10-bit accuracy, a programmable Watchdog Timer with Internal Oscillator, an SPI serial port, and five software selectable power saving modes. The Idle mode stops the CPU while allowing the SRAM, Timer/Counters, SPI port, and interrupt system to continue functioning. The Powerdown mode saves the register contents but freezes the Oscillator, disabling all other chip functions until the next Interrupt or Hardware Reset. In Power-save mode, the asynchronous timer continues to run, allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC Noise Reduction mode stops the CPU and all I/O modules except asynchronous timer and ADC, to minimize switching noise during ADC conversions. In Standby mode, the crystal/resonator Oscillator is running while the rest of the device is sleeping. This allows very fast start-up combined with low-power consumption. The device is manufactured using Atmel’s high density non-volatile memory technology. The Flash Program memory can be reprogrammed In-System through an SPI serial interface, by a conventional non-volatile memory programmer, or by an On-chip boot program running on the AVR core. The boot program can use any interface to download the application program in the Application Flash memory. Software in the Boot Flash Section will continue to run while the Application Flash Section is updated, providing true Read-While-Write operation. By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip, the Atmel ATmega8 is a powerful microcontroller that provides a highly-flexible and cost-effective solution to many embedded control applications. The ATmega8 AVR is supported with a full suite of program and system development tools, including C compilers, macro assemblers, program debugger/simulators, In-Circuit Emulators, and evaluation kits. Typical values contained in this datasheet are based on simulations and characterization of other AVR microcontrollers manufactured on the same process technology. Min and Max values will be available after the device is characterized. 4 ATmega8(L) 2486T–AVR–05/08 ATmega8(L) Pin Descriptions VCC Digital supply voltage. GND Ground. Port B (PB7..PB0) XTAL1/XTAL2/TOSC1/ TOSC2 Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri-stated when a reset condition becomes active, even if the clock is not running. Depending on the clock selection fuse settings, PB6 can be used as input to the inverting Oscillator amplifier and input to the internal clock operating circuit. Depending on the clock selection fuse settings, PB7 can be used as output from the inverting Oscillator amplifier. If the Internal Calibrated RC Oscillator is used as chip clock source, PB7..6 is used as TOSC2..1 input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set. The various special features of Port B are elaborated in “Alternate Functions of Port B” on page 58 and “System Clock and Clock Options” on page 25. Port C (PC5..PC0) Port C is an 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up resistors are activated. The Port C pins are tri-stated when a reset condition becomes active, even if the clock is not running. PC6/RESET If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical characteristics of PC6 differ from those of the other pins of Port C. If the RSTDISBL Fuse is unprogrammed, PC6 is used as a Reset input. A low level on this pin for longer than the minimum pulse length will generate a Reset, even if the clock is not running. The minimum pulse length is given in Table 15 on page 38. Shorter pulses are not guaranteed to generate a Reset. The various special features of Port C are elaborated on page 61. Port D (PD7..PD0) Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up resistors are activated. The Port D pins are tri-stated when a reset condition becomes active, even if the clock is not running. Port D also serves the functions of various special features of the ATmega8 as listed on page 63. RESET Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running. The minimum pulse length is given in Table 15 on page 38. Shorter pulses are not guaranteed to generate a reset. 5 2486T–AVR–05/08 AVCC AREF ADC7..6 (TQFP and QFN/MLF Package Only) AVCC is the supply voltage pin for the A/D Converter, Port C (3..0), and ADC (7..6). It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that Port C (5..4) use digital supply voltage, VCC. AREF is the analog reference pin for the A/D Converter. In the TQFP and QFN/MLF package, ADC7..6 serve as analog inputs to the A/D converter. These pins are powered from the analog supply and serve as 10-bit ADC channels. 6 ATmega8(L) 2486T–AVR–05/08 ATmega8(L) Resources A comprehensive set of development tools, application notes and datasheets are available for download on http://www.atmel.com/avr. Note: 1. Data Retention Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM over 20 years at 85°C or 100 years at 25°C. 7 2486T–AVR–05/08 About Code Examples This datasheet contains simple code examples that briefly show how to use various parts of the device. These code examples assume that the part specific header file is included before compilation. Be aware that not all C compiler vendors include bit definitions in the header files and interrupt handling in C is compiler dependent. Please confirm with the C compiler documentation for more details. 8 ATmega8(L) 2486T–AVR–05/08 ATmega8(L) AVR CPU Core Introduction Architectural Overview This section discusses the AVR core architecture in general. The main function of the CPU core is to ensure correct program execution. The CPU must therefore be able to access memories, perform calculations, control peripherals, and handle interrupts. Figure 2. Block Diagram of the AVR MCU Architecture Data Bus 8-bit Flash Program Memory Instruction Register Instruction Decoder Control Lines Program Counter Status and Control 32 x 8 General Purpose Registrers ALU Interrupt Unit SPI Unit Watchdog Timer Analog Comparator Direct Addressing Indirect Addressing Data SRAM EEPROM i/O Module1 i/O Module 2 i/O Module n I/O Lines In order to maximize performance and parallelism, the AVR uses a Harvard architecture – with separate memories and buses for program and data. Instructions in the Program memory are executed with a single level pipelining. While one instruction is being executed, the next instruction is pre-fetched from the Program memory. This concept enables instructions to be executed in every clock cycle. The Program memory is In-System Reprogrammable Flash memory. The fast-access Register File contains 32 x 8-bit general purpose working registers with a single clock cycle access time. This allows single-cycle Arithmetic Logic Unit (ALU) operation. In a typical ALU operation, two operands are output from the Register File, the operation is executed, and the result is stored back in the Register File – in one clock cycle. Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing – enabling efficient address calculations. One of the these address pointers 9 2486T–AVR–05/08 can also be used as an address pointer for look up tables in Flash Program memory. These added function registers are the 16-bit X-, Y-, and Z-register, described later in this section. The ALU supports arithmetic and logic operations between registers or between a constant and a register. Single register operations can also be executed in the ALU. After an arithmetic operation, the Status Register is updated to reflect information about the result of the operation. The Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the whole address space. Most AVR instructions have a single 16-bit word format. Every Program memory address contains a 16- or 32-bit instruction. Program Flash memory space is divided in two sections, the Boot program section and the Application program section. Both sections have dedicated Lock Bits for write and read/write protection. The SPM instruction that writes into the Application Flash memory section must reside in the Boot program section. During interrupts and subroutine calls, the return address Program Counter (PC) is stored on the Stack. The Stack is effectively allocated in the general data SRAM, and consequently the Stack size is only limited by the total SRAM size and the usage of the SRAM. All user programs must initialize the SP in the reset routine (before subroutines or interrupts are executed). The Stack Pointer SP is read/write accessible in the I/O space. The data SRAM can easily be accessed through the five different addressing modes supported in the AVR architecture. The memory spaces in the AVR architecture are all linear and regular memory maps. A flexible interrupt module has its control registers in the I/O space with an additional global interrupt enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector position. The lower the Interrupt Vector address, the higher the priority. The I/O memory space contains 64 addresses for CPU peripheral functions as Control Registers, SPI, and other I/O functions. The I/O Memory can be accessed directly, or as the Data Space locations following those of the Register File, 0x20 - 0x5F. 10 ATmega8(L) 2486T–AVR–05/08 ATmega8(L) Arithmetic Logic Unit – ALU Status Register The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers. Within a single clock cycle, arithmetic operations between general purpose registers or between a register and an immediate are executed. The ALU operations are divided into three main categories – arithmetic, logical, and bit-functions. Some implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and fractional format. See the “Instruction Set” section for a detailed description. The Status Register contains information about the result of the most recently executed arithmetic instruction. This information can be used for altering program flow in order to perform conditional operations. Note that the Status Register is updated after all ALU operations, as specified in the Instruction Set Reference. This will in many cases remove the need for using the dedicated compare instructions, resulting in faster and more compact code. The Status Register is not automatically stored when entering an interrupt routine and restored when returning from an interrupt. This must be handled by software. The AVR Status Register – SREG – is defined as: Bit Read/Write Initial Value 7 6 5 4 3 2 1 0 I T H S V N Z C SREG R/W R/W R/W R/W R/W R/W R/W R/W 0 0 0 0 0 0 0 0 • Bit 7 – I: Global Interrupt Enable The Global Interrupt Enable bit must be set for the interrupts to be enabled. The individual interrupt enable control is then performed in separate control registers. If the Global Interrupt Enable Register is cleared, none of the interrupts are enabled independent of the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by the RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by the application with the SEI and CLI instructions, as described in the Instruction Set Reference. • Bit 6 – T: Bit Copy Storage The Bit Copy instructions BLD (Bit LoaD) and BST (Bit STore) use the T-bit as source or destination for the operated bit. A bit from a register in the Register File can be copied into T by the BST instruction, and a bit in T can be copied into a bit in a register in the Register File by the BLD instruction. • Bit 5 – H: Half Carry Flag The Half Carry Flag H indicates a Half Carry in some arithmetic operations. Half Carry is useful in BCD arithmetic. See the “Instruction Set Description” for detailed information. • Bit 4 – S: Sign Bit, S = N ⊕ V The S-bit is always an exclusive or between the Negative Flag N and the Two’s Complement Overflow Flag V. See the “Instruction Set Description” for detailed information. • Bit 3 – V: Two’s Complement Overflow Flag The Two’s Complement Overflow Flag V supports two’s complement arithmetics. See the “Instruction Set Description” for detailed information. • Bit 2 – N: Negative Flag The Negative Flag N indicates a negative result in an arithmetic or logic operation. See the “Instruction Set Description” for detailed information. • Bit 1 – Z: Zero Flag The Zero Flag Z indicates a zero result in an arithmetic or logic operation. See the “Instruction Set Description” for detailed information. 11 2486T–AVR–05/08 General Purpose Register File • Bit 0 – C: Carry Flag The Carry Flag C indicates a Carry in an arithmetic or logic operation. See the “Instruction Set Description” for detailed information. The Register File is optimized for the AVR Enhanced RISC instruction set. In order to achieve the required performance and flexibility, the following input/output schemes are supported by the Register File: • One 8-bit output operand and one 8-bit result input. • Two 8-bit output operands and one 8-bit result input. • Two 8-bit output operands and one 16-bit result input. • One 16-bit output operand and one 16-bit result input. Figure 3 shows the structure of the 32 general purpose working registers in the CPU. Figure 3. AVR CPU General Purpose Working Registers General Purpose Working Registers 7 0 R0 R1 R2 … R13 R14 R15 R16 R17 … R26 R27 R28 R29 R30 R31 Addr. 0x00 0x01 0x02 0x0D 0x0E 0x0F 0x10 0x11 0x1A 0x1B 0x1C 0x1D 0x1E 0x1F X-register Low Byte X-register High Byte Y-register Low Byte Y-register High Byte Z-register Low Byte Z-register High Byte Most of the instructions operating on the Register File have direct access to all registers, and most of them are single cycle instructions. As shown in Figure 3, each register is also assigned a Data memory address, mapping them directly into the first 32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory organization provides great flexibility in access of the registers, as the X-, Y-, and Z-pointer Registers can be set to index any register in the file. 12 ATmega8(L) 2486T–AVR–05/08 ATmega8(L) The X-register, Y- The registers R26..R31 have some added functions to their general purpose usage. These reg- register and Z-register isters are 16-bit address pointers for indirect addressing of the Data Space. The three indirect address registers X, Y and Z are defined as described in Figure 4. Figure 4. The X-, Y- and Z-Registers 15 XH XL 0 X-register 7 07 0 R27 (0x1B) R26 (0x1A) 15 YH YL 0 Y-register 7 07 0 R29 (0x1D) R28 (0x1C) Z-register 15 ZH 7 0 R31 (0x1F) ZL 0 7 0 R30 (0x1E) In the different addressing modes these address registers have functions as fixed displacement, automatic increment, and automatic decrement (see the Instruction Set Reference for details). Stack Pointer The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that the Stack is implemented as growing from higher memory locations to lower memory locations. This implies that a Stack PUSH command decreases the Stack Pointer. The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. This Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or interrupts are enabled. The Stack Pointer must be set to point above 0x60. The Stack Pointer is decremented by one when data is pushed onto the Stack with the PUSH instruction, and it is decremented by two when the return address is pushed onto the Stack with subroutine call or interrupt. The Stack Pointer is incremented by one when data is popped from the Stack with the POP instruction, and it is incremented by two when address is popped from the Stack with return from subroutine RET or return from interrupt RETI. The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is implementation dependent. Note that the data space in some implementations of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register will not be present. Bit 15 14 13 12 11 10 9 8 SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL 7 6 5 4 3 2 1 0 Read/Write R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W Initial Value 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Instruction Execution Timing This section describes the general access timing concepts for instruction execution. The AVR CPU is driven by the CPU clock clkCPU, directly generated from the selected clock source for the chip. No internal clock division is used. 13 2486T–AVR–05/08 Figure 5 shows the parallel instruction fetches and instruction executions enabled by the Harvard architecture and the fast-access Register File concept. This is the basic pipelining concept to obtain up to 1 MIPS per MHz with the corresponding unique results for functions per cost, functions per clocks, and functions per power-unit. Figure 5. The Parallel Instruction Fetches and Instruction Executions T1 T2 T3 T4 clkCPU 1st Instruction Fetch 1st Instruction Execute 2nd Instruction Fetch 2nd Instruction Execute 3rd Instruction Fetch 3rd Instruction Execute 4th Instruction Fetch Figure 6 shows the internal timing concept for the Register File. In a single clock cycle an ALU operation using two register operands is executed, and the result is stored back to the destination register. Figure 6. Single Cycle ALU Operation T1 T2 T3 T4 clkCPU Total Execution Time Register Operands Fetch ALU Operation Execute Result Write Back Reset and Interrupt Handling The AVR provides several different interrupt sources. These interrupts and the separate Reset Vector each have a separate Program Vector in the Program memory space. All interrupts are assigned individual enable bits which must be written logic one together with the Global Interrupt Enable bit in the Status Register in order to enable the interrupt. Depending on the Program Counter value, interrupts may be automatically disabled when Boot Lock Bits BLB02 or BLB12 are programmed. This feature improves software security. See the section “Memory Programming” on page 222 for details. The lowest addresses in the Program memory space are by default defined as the Reset and Interrupt Vectors. The complete list of Vectors is shown in “Interrupts” on page 46. The list also determines the priority levels of the different interrupts. The lower the address the higher is the priority level. RESET has the highest priority, and next is INT0 – the External Interrupt Request 0. The Interrupt Vectors can be moved to the start of the boot Flash section by setting the Interrupt Vector Select (IVSEL) bit in the General Interrupt Control Register (GICR). Refer to “Interrupts” on page 46 for more information. The Reset Vector can also be moved to the start of the boot Flash section by programming the BOOTRST Fuse, see “Boot Loader Support – ReadWhile-Write Self-Programming” on page 209. 14 ATmega8(L) 2486T–AVR–05/08 ATmega8(L) When an interrupt occurs, the Global Interrupt Enable I-bit is cleared and all interrupts are disabled. The user software can write logic one to the I-bit to enable nested interrupts. All enabled interrupts can then interrupt the current interrupt routine. The I-bit is automatically set when a Return from Interrupt instruction – RETI – is executed. There are basically two types of interrupts. The first type is triggered by an event that sets the Interrupt Flag. For these interrupts, the Program Counter is vectored to the actual Interrupt Vector in order to execute the interrupt handling routine, and hardware clears the corresponding Interrupt Flag. Interrupt Flags can also be cleared by writing a logic one to the flag bit position(s) to be cleared. If an interrupt condition occurs while the corresponding interrupt enable bit is cleared, the Interrupt Flag will be set and remembered until the interrupt is enabled, or the flag is cleared by software. Similarly, if one or more interrupt conditions occur while the global interrupt enable bit is cleared, the corresponding Interrupt Flag(s) will be set and remembered until the global interrupt enable bit is set, and will then be executed by order of priority. The second type of interrupts will trigger as long as the interrupt condition is present. These interrupts do not necessarily have Interrupt Flags. If the interrupt condition disappears before the interrupt is enabled, the interrupt will not be triggered. When the AVR exits from an interrupt, it will always return to the main program and execute one more instruction before any pending interrupt is served. Note that the Status Register is not automatically stored when entering an interrupt routine, nor restored when returning from an interrupt routine. This must be handled by software. When using the CLI instruction to disable interrupts, the interrupts will be immediately disabled. No interrupt will be executed after the CLI instruction, even if it occurs simultaneously with the CLI instruction. The following example shows how this can be used to avoid interrupts during the timed EEPROM write sequence. Assembly Code Example in r16, SREG ; store SREG value cli ; disable interrupts during timed sequence sbi EECR, EEMWE ; start EEPROM write sbi EECR, EEWE out SREG, r16 ; restore SREG value (I-bit) C Code Example char cSREG; cSREG = SREG; /* store SREG value */ /* disable interrupts during timed sequence */ _CLI(); EECR |= (1<